The invention relates to a displacement transducer for identifying the axial position of a pressing plunger of a glass forming machine. The pressing plunger is mounted on a piston rod of a piston-cylinder unit for pressing a gob of molten glass into a parison mold. The transducer includes an annular coil fixed relative to the cylinder and a metallic actuating member mounted on the piston. Moving with the piston, the actuating member cooperates with the annular coil to change the inductance of the coil in dependence upon the axial relative position of the piston and of the cylinder. The coil is arranged in the region of one end of the cylinder and encircles the piston rod. The piston rod extends through this end of the cylinder and is guided by guide sleeves. The actuating member is annular and arranged on the side of the piston facing the pressing plunger. Connected to the coil, an electrical evaluation circuit identifies the axial position of the pressing plunger.
In one known displacement transducer of this type (DE 34 01 465 Cl) the coil 19 is arranged coaxially with respect to the piston rod 4 between the guide sleeve 25 and the piston 7. The actuating member 27 is fixed on the side of the piston facing the coil 19 and concentrically in relation to the piston rod 4, and protrudes into the coil 19. This structure requires a comparatively large axial amount of room. Consequently, on the one hand the axial displacement detection zone is limited and on the other hand a subsequent incorporation of the displacement transducer into existing piston-cylinder units is in many cases not possible. The cyclical temperature changes which arise by virtue of the process can lead to the coil coming apart from its support. The manufacture of the annular holder 18 with the coil 19 is very complex and expensive.
From DE 40 31 931 Al there is known an inductive length-measuring sensor 1 which serves to improve the linearity of or to increase the stroke of a differential transformer 15, 35, 36 which has an electronic evaluation circuit connected to the secondary winding 35, 36. The stationary primary winding 15 is located at the center longitudinally, and on the outside of a tubular inner part 12 of a stationary ferromagnetic short-circuit device 11, whose outer part 13 is formed as a thin-walled tube in a bore 5 of a base member 2 made of brass. The inner part 12 is supported on a tubular carrier 6 which is fixed in a bore 4 of the base member 2. Between the parts 12, 13 there is an annular air gap 24 in which a tubular collar 25 of a cup-shaped coil carrier 26 protrudes. On the outside of the coil carrier 26 are embedded part-windings 35, 36 of the secondary winding, axially spaced from one another, so that the part-windings 35, 36 are each located, in an axial mid-position of the coil carrier 26, at the same axial spacing from the stationary primary winding 15. A base 27 of the coil carrier 26 is fixed to a concentric sensing rod 28 which extends through the tubular carrier 6 and is radially guided beyond the ends of the carrier 6 by bearing sleeves 39, 41. Connecting wires 17 serve for the connection of the stationary primary winding 15 to an ac voltage source. Connecting wires 37 to the two part-windings 35, 36 of the displaceable secondary winding lead to an electronic evaluation circuit with a high-resistance input and a phase-sensitive rectifier device.
From DE 31 09 930 Al is known a displacement transducer for hydraulic elements, such as piston 7 and cylinder 8, which are subject to temperature variations. The object is to reduce zero-point drift. An electrically non-conducting rod core 1 is fixed relative to the cylinder 8, carries a single-layer cylinder coil 4 fed with high frequency alternating current through leads 5, 6 and extends through the piston 7 over the full piston stroke into an internal chamber of the piston rod 2. Piston 7 and piston rod 2 are sheathed in the region of the rod core 1 and its coil 4 with a diamagnetic sleeve 3 which is a good electrical conductor, preferably of brass or copper.
It is the object of the invention to broaden the range of application of the displacement transducer, to increase its life and to make possible a greater distance over which the position of the piston can be identified, with the same dimensions of the overall mechanism, as well as to make it possible to manufacture the product at a more favorable cost.